Controlled Shape Evolution of Pure-MOF 1D Microcrystals towards Efficient Waveguide and Laser Applications

Article abstract of DOI:10.1002/chem.202005217

MOF-based one-dimensional materials have received increasing attention in the nanophotonics field, but it is still difficult in the flexible shape evolution of MOF micro/nanocrystals for desired optical functionalities due to the susceptible solvothermal

Full text of DOI:10.1002/chem.202005217

Accepted Article
Accepted Article
Title: Controlled Shape–Evolution of Pure-MOF 1D-Microcrystals
towards Efficient Waveguide and Laser Applications
Authors: Yuanchao Lv, Zhile Xiong, Yinan Yao, Ang Ren, Shengchang
Xiang, Yong Sheng Zhao, and Zhangjing Zhang
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To be cited as: Chem. Eur. J. 10.1002/chem.202005217
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1/2020

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Controlled Shape–Evolution of Pure-MOF 1D-Microcrystals
towards Efficient Waveguide and Laser Applications
Yuanchao Lv,*,+,[a] Zhile Xiong,+,[a] Yinan Yao, Ang Ren, Shengchang Xiang, Yong Sheng Zhao,*
[b]
[b]
[a]
,[b]
and Zhangjing Zhang*,[a]
[
a]
Dr. Y. Lv, Z. Xiong, Prof. S. Xiang, Prof. Z. Zhang
Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University
Fuzhou 350007, China
E-mail: lvyc@fjnu.edu.cn; zzhang@fjnu.edu.cn
[
b]
Y. Yao, A. Ren, Prof. Y. S. Zhao
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190, China
E-mail: yszhao@iccas.ac.cn
These authors contributed equally to this work.
[+
]
Supporting information for this article is given via a link at the end of the document.
Abstract: MOF-based one-dimensional materials have received
different 1D-nanomaterials, including microrods, microtubes, and
nanowires.[ In addition, tailorable coordination bonds of MOFs
make it possible to control the morphology evolution and even to
7]
increasing attention in nanophotonics field, but it is still diffcult in the
flexible shape–evolution of MOF micro/nanocrystals for desired
optical functionalities due to the susceptible solvothermal growth
process. Herein, we report on the well-controlled shape–evolution of
pure-MOF microcrystals with optical waveguide and lasing
performances based on a bottom-up & top-down synergistic method.
The MOF microcrystals from solvethermol synthesis (bottom-up)
enable to evolve from microrods via microtubes to nanowires through
a chelating agent-assisted etching process (top-down). The three
types of MOF 1D-microstructures with high crystallinity and smooth
surfaces all exhibit efficient optical waveguide performance.
Furthermore, MOF nanowire with lowest propagation loss served as
low-threshold pure-MOF nanolasers with Fabry−Pérot resonance.
These results advance the fundamental understanding on the
controlled MOF evolution mechanism, and offer a valuable route for
the development of pure-MOF-based photonic components with
desired functionalities.
[
8]
achieve multilevel structures for unique photonic functionalities.
Moreover, the excellent compatibility of MOFs can ensure the
[
9]
incorporation of organic gain blocks into MOF 1D-materials,
showing significant potential for performing low-loss optical
waveguide and efficient lasing behaviors.[
10]
Therefore, the
development of MOF 1D-microcrystals with controlled shape–
evolution could provide an innovative guidance to realize robust
pure-MOF photonic components with desired waveguide/lasing
performances. However, it is still diffcult in the shape–evolution of
MOF micro/nanocrystals for desired optical functionalities due to
the susceptible solvothermal growth process.
In this work, we propose an approach to realize shape–
evolution of pure-MOF microcrystals with optical waveguide and
lasing performances based on
a bottom-up & top-down
synergistic method. The MOF microcrystals from solvethermol
synthesis (bottom-up) can successfully evolve from microrods via
microtubes to nanowires via a chelating agent-assisted etching
process (top-down). The three types of MOF 1D-microstructures
with high crystallinity and smooth surfaces all display efficient
optical waveguide performances. Moreover, MOF nanowire with
lowest propagation loss functioned as Fabry−Pérot (FP) pure-
MOF nanolasers with low threshold. These results provide deep
insights into the mechanism of MOF growth and evolution, and
open a valuable path for the exploration of pure-MOF-based
photonic devices with on-demand waveguide/lasing functions.
Here, the MOF crystal of tpbe-Cd, assembled by tpbe (1,1,2,2-
tetrakis(4'-(pyridin-4-yl)-[1,1'-biphenyl]-4-yl) ethene) as organic
linker and Cd2+ ion as metal node (Table S1), was selected as the
model MOF to construct the pure-MOF 1D crystals with evolvable
shape as desired photonic component for the following merits. (i)
The tpbe molecule exhibits the efficient aggregation-induced
emission characteristic, which endow tpbe-Cd with high optical
gain (Figure 1a and S1).[ (ii) The framework of tpbe-Cd contains
a kind of metal chain (Figure 1b and c), which facilitate the growth
of tpbe-Cd along a specific direction. Indeed, the predicted
growth of tpbe-Cd confirm the 1D morphology (Figure 1d),
showing potential for the optical waveguide or resonance
behavior. (iii) tpbe-Cd possesses the large 1D channel with
aperture of 12.9 Å (Figure S2), which favor the absorption of the
chelating agent to trigger the chelation-assisted etching at
One-dimensional (1D) nanomaterials have attracted great
attentions in both fundamental study and potential optoelectronic
application due to the capacity of propagating and manipulating
light efficiently in (sub)wavelength scale.[ Up to now, great efforts
have been paid to develop 1D inorganic and organic
nanomaterials to realize superior physical and chemical
properties.[2] For example, various 1D nanomaterials, such as
nanowires, nanotubes and microrobs, have been extensively
applied as the ideal building block in optical waveguides,
micro/nanolasers, and other optoelectronic fields.[ While, most
of 1D inorganic nanomaterials are fabricated via the top-down
method, which usually requires high temperature or the
sophisticated fabrication techniques. Compared with inorganic
counterparts, 1D organic nanomaterials can be obtained by
simple self-assembly because of their excellent flexibility and
1]
3]
[
4]
processability. However, organic materials suffer poor optical
instability, limiting their potential applications in optoelectronic
devices.
11]
Metal−organic frameworks (MOFs), as typical crystalline
[
5]
materials with inorganic-organic hybrid properties, not only
possesses great stability but also displays outstanding
processability on shape, size and pore diameter, emerging as a
promising tool kit for robust optoelectronic materials.[6] These
MOFs with designable framework geometry allow for engineering
1
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method of tpbe-Cd microcrystals. f, g, h) Bright field microscopy images of
microrods, microtubes and nanowires. Scale bar: 20 μm. Inset: SEM images of
the tips of samples. Scale bar: 10 μm.
selective crystal facets, possibly permitting the MOF shape-
evolution for desired photonic functions.[12] In addition, the tpbe-
Cd exhibited the relatively high stability, which is favorable for the
practical application.
Although the shape-evolution of MOF crystals can be
accomplished via the templated synthesis or size-confinement
effect,[13] these methods usually induce inevitable damages to the
crystallinity and surface roughness of MOF microcrystals, leading
to unexpected optical defects. Here, we propose a synergistic
method of the bottom-up and top-down processes to fabricate
tpbe-Cd microcrystals with designable shapes, which evolve from
microrobs via microtubes to nanowires (Figure 1e). In a typical
preparation (see Supporting Information for details), the
To controllably modulate the shape-evolution of tpbe-Cd
microcrystals for desired photonic functions, we investigated the
etching process by monitoring their temporal morphology over
time. At the initial stage, the well-defined microrods were exposed
to EDTA molecules (Figure 2a-i). Then, after ~4 h, small holes
appeared on the end facet of microrods, which confirmed the
beginning of etching behavior (Figure 2a-ii). With the reaction
prolonged to 11 hours, the microrods with small holes totally
evolve into microtubes (Figure 2a-iii). Interestingly, with the
reaction time further increasing, the cracked microtubes are
observed (Figure 2a-iv), indicating that etching process was
transferred from the center of microrods to the walls of microtubes.
Finally, the microtubes break up into nanowires after a sampling
time of 24h (Figure 2a-v).
2 2
solvothermal reaction of tpbe and Cd(NO3) ꞏ4H O in solution of
DMAC/H2O yielded tpbe-Cd microcrystals with rod-like shape.
Then, the chelating agent (ethylene diamine tetraacetic acid,
EDTA) was added to the solution. The EDTA, taking advantage
of the chelation with metal ions, would break the coordination
bonds between tpbe and Cd2+ ion to tailor the shape of tpbe-Cd
crystals, leading to the generation of other two kinds of
morphologies (microtube and nanowire).[12] On the whole, the
bottom-up process is based on the solvothemal process (the
formation of coordinate bond) and the top-down process is the
etching process activated by the chelating agent (the destruction
of coordinate bond).
On the basis of the above observations, we put forward an
evolution mechanism of these tpbe-Cd microcrystals (Figure 2a).
The end facets of microrods with high attachment energies play
the role of condensation center to facilitate the adsorption of
EDTA molecules, and then trigger the etching process on end
facets.[ These MOF microrods with great porosity (56.9%)
enables the EDTA molecules to intrude into the rob interior, and
thus were etched as hollow microtubes. Subsequently, the
stacking direction of coordination bonds induce the etching to
continue along the a axis on tubular walls, which makes
microtubes split. Finally, after exhaustion of EDTA molecules,
microtubes completely turned into nanowires. Following the
above evolution mechanism, we can construct tpbe-Cd
microrods, microtubes and nanowires by controlling the
14]
As a result, the evolution of three kinds of MOF-based
microcrystals, including microrod, microtube, and nanowire, were
obtained. As shown in Figure 1f,
g and h, these MOF
microcrystals all possess the well-defined 1D profiles, possibly
triggering applications in 1D waveguide or optical confinement.
Especially, the typical channel structures of the microtube can be
readily observed from the bright-field microscopy image (Figure
1g). The scanning-electron-microscopy (SEM) images (Figure 1f,
−
1
concentration of EDTA molecules at 0, 2.7, and 6.2 mg mL .
g and h, inset) revealed that the MOF microrods, microtubes, and
nanowires have the smooth and flat surfaces, which would
effectively reduce the optical scattering loss and allow the
feedback of guide emission, which is highly favorable for optical
gain and amplification actions.
Figure 2. a) Schematic diagram for the etching process of the tpbe-Cd
microcrystal. Insert: SEM images of intermediates at different etching stages: (i)
0
h; (ii) 4 h; (iii) 11 h; (iv) 16 h; (v) 24 h. Scale bar: 10 μm. b, c, d) PL images of
the microrods, microtubes and nanowires. Scale bar: 10 μm.
The XRD spectra in Figure S3 illustrated that these MOF
microrods, microtubes and nanowires are highly crystalline, and
belong to the same crystalline phase, which confirmed the
evolution mechanism. These crystalline MOF microstructures,
under UV (330-380 nm) excitation, showed the brighter spots from
Figure 1. a, b) Structural representations of the tpbe ligand and the metal chain.
c) Structural representation of tpbe-Cd. d) Simulated growth morphology of
tpbe-Cd. e) Schematic representation for the shape–evolution synergistic
2
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tips and weaker emission on the bodies (Figure 2b, c and d). This
feature indicates that these MOF microstructures display the
typical characteristics of optical waveguide. Moreover, microrods,
microtubes, and nanowires perform the similar blue-green
emission with peak of 490 nm, consistent with that of tpbe-Cd
resulting in low propagation loss. While, compared with the
nanowire, the MOF microtube exhibited somewhat higher optical
loss, which be attributed to its larger cross section, leading to
more unexpected scattering defects. Therefore, among these
samples, the MOF nanowires demonstrate the lowest
propagation loss, making it possible to achieve efficient optical
resonance for low-threshold lasing.
(Figure S4), revealing that tpbe-Cd microcrystals still keep the
intrinsic optical properties during the evolving process.
These MOF microstructures with ideal 1D shape and
spontaneous emission character enable us to investigate their
optical waveguide properties. The spatially-resolved PL imaging
and spectra measurements were performed by locally exciting a
single sample with a 400-nm laser beam. Figure 3a, b and c
illustrate the micro-area PL images taken from a microrod, a
microtube, and a nanowire, respectively, via accurately moving
the excitation laser spots. We can clearly observe the PL light
guided to the far ends as a bright spot (marked in Figure 3a, b and
c), indicating the nature of the active optical waveguide in these
MOF samples. The corresponding PL signals were obtained from
the tips with different photon guiding distance (Figure 3d, e and f).
The optical loss coefficient (α) could be calculated through the
function, log(Itip/Ibody) = −αD, in which Itip and Ibody represent PL
intensities of the emitting tip and excited site, respectively, and D
represents the guiding distance between the two sites.
These MOF nanowires with low propagation loss and active
optical properties provide a superior system for studying the
lasing performance. Here, the MOF nanowire was excited by a
400 nm pulsed laser beam (~150 fs) in a homebuilt micro-PL
system (Figure S5). With the pump fluence increasing, a set of
sharp optical-mode peaks around 490 nm emerged in PL spectra
from the tips, and was dramatically amplified (Figure 4a). The
corresponding PL intensities dependence on pump fluence
perform a clear S-shaped nonlinear behavior at threshold of ~522
nJ cm-2 (Figure 4b), much lower than reported pure-MOF sapmles,
which demonstrates a three-stage transition with spontaneous
emission, amplified spontaneous emission, and the full lasing
oscillation, and thus revealed the occurrence of lasing in the MOF
nanowire.[15]
Figure 4. a) Spectra of the tpbe-Cd nanowire with different pump fluences.
Inset: Bright-field and PL images (below and above threshold) of the nanowire.
Scale bar: 20 μm. b) The values of emission intensities and FWHM against the
pump fluence. c) The spectra of nanowires with varied L. Scale bar: 20 μm. d)
Figure 3. a, b, c) Bright-field and PL images obtained from the tpbe-Cd
microrod, microtube and nanowire excited at different positions. Scale bar: 20
μm. d, e, f) The corresponding spectra from the right tips of samples with
different guiding distances. g, h, i) The plot of log(Itip/Ibody) against the guiding
distance.
2
The values of λ /2Δλ (λ = 490 nm) against L.
The PL image of the MOF nanowire above threshold showed
impressive emissions on two tips (Figure 4a, insert), suggesting
the Fabry−Pérot (FP) resonance in the nanowire (Figure S6). The
lasing spectra of MOF nanowires with varied lengths were
measured to study the optical-cavity effect. The mode number of
lasing signals gradually increased with increasing the nanowire
length (L) (Figure 4c). The change on mode spacing (∆λ) against
Accordingly, αmicrorod = 0.0081 dB μm , αmicrotube = 0.0064 dB μm-
-1
1
and αnanowire = 0.0041 dB μm-1 at 490 nm, much lower than that
of reported organic waveguide materials (Figure 3g, h and i). This
merit benefit from the high crystallinity and smooth surfaces of
these MOF microcrystals. Notably, the MOF microtube show
lower optical loss than the microrod, which attribute to the tubular
structure. According to the great distinction of the refractive index
between tpbe-Cd (n = ~2.85, Figure 4d) and air (nair = 1), the air
in the hollow induced strong 2D spatial optical confinement in
tubular walls, which would largely reduce the interaction of
photons reflected from internal surfaces with the substrate,
2
the nanowire length conformed to the FP-theory equation, λ /2∆λ
=
nL, where n represents the refractive index, indicating that PL
modulation originates from FP-type resonance (Figure 4d). The
derived parameter n around 490 nm is ~2.85, which is high
enough to ensure effective optical confinement in nanowires.
3
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3
These MOF nanowries exhibited the quality factors (Q) of ~10
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Figure S7), pretty high for MOF-based microcavities.[10a]
In summary, we report the shape–evolution of pure-MOF
microcrystals with optical waveguide and lasing performances
through a bottom-up & top-down synergistic method. The
solvethermol MOF microcrystals (bottom-up) have successfully
evolved from microrods via microtubes to nanowires through a
chelating agent-assisted etching process (top-down). The three
kinds of MOF microstructures with high crystallinity and smooth
surfaces all process great optical waveguide properties.
Furthermore, MOF nanowire with the lowest propagation loss
served as low-threshold FP-type pure-MOF nanolasers. These
results provide a comprehensive investigation on the MOF growth
mechanism, and inspire the rational design of pure-MOF
nanophotonics devices with unique functionalities.
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Keywords: metal−organic framework • shape evolution •
[
nanophotonics • microlasers
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4
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Entry for the Table of Contents
The shape–evolution of pure-MOF 1D-microcrystals with ideal nanophotonics functions was firstly achieved based on a bottom-
up/top-down synergistic method. These microcrystals enable to evolve from microrods via microtubes to nanowires with efficient
optical waveguide and laser applications.
5
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